Seismically induced kinking in quartz

Deformed quartz veins next (1-1.5m) to an exhumed pseudotachylyte-bearing (i.e. anciently seismic) fault within the Schobergruppe (Austroalpine Crystalline Complex, Eastern Alps) contain intensely kinked quartz grains. In general, kinking requires the presence of a planar mechanical anisotropy, e.g. a multi-layered structure with a regular periodic alternation of thin weakly bounded layers (or of high viscosity layers interleaved with thin low viscosity ones) such as in minerals with a strong cleavage, e.g. micas or in industrial metallic nano-laminates. Since quartz does not commonly have a strong mechanical anisotropy, we address the question of why and how kinking of quartz may develop during the seismic cycle.

The monoclinic symmetry of kink bands is consistent with the slip sense of the fault. Cathodoluminescence images show a very high density of intragranular, sub-planar, lamellae accompanied by nanometre-scale fluid-related porosity visible in electron backscatter orientation contrast. Based on the oscillating orientation variation across subgrain boundaries (misorientation angle 1-9°) these lamellae (oriented (sub)parallel to a rhomb plane and spaced 4-10 µm apart) are identified as short-wavelength undulatory extinction microstructures (SWUE). Transmission electron microscopy reveals a high degree of recovery (low dislocation density) across the SWUE. Only grains with SWUE oriented parallel to the vein boundary are kinked. We infer following history for the kink evolution related to the seismic cycle: (I) Deformation lamellae formed during high differential stresses preceding the earthquake rupturing or associated with seismic rupture propagation. The initial high dislocation density within the deformation lamellae provided the mechanical anisotropy in quartz required for (II) the subsequent coseismic initiation of kinking. The lamellae acted as a geometric filter that only allowed r<a> slip of dislocations parallel to the lamellae. These athermal dislocations were able to glide fast over a relatively large distance before piling up and initiating kinking during the coseismic event. Progressive build-up of dislocations resulted in deformation bands which accumulated the final misorientation angle between host domain and kink domain. (III) During post-seismic deformation dislocations were dynamically re-arranged under residual stress into sub-parallel subgrain boundaries which now characterize the kink band boundary region. We suggest that kinking in quartz potentially indicates coseismic deformation and is an important mechanism for incipient strain accommodation during high strain rates.